1. Field of the Invention
This invention relates to an idling speed control system for an internal combustion engine.
2. Description of the Prior Art
In an internal combustion engine of a vehicle, it is generally preferred to maintain the idling speed of the engine at a low value of about 600 to 700 rpm from the viewpoint of fuel consumption. For example, in U.S. Pat. No. 3,661,131 is disclosed a system for maintaining a stabilized idling speed without any possiblity of the engine's stalling in which the actual rotational speed of the engine is detected to carry out a feedback control so that the actual rotational speed during idling is equalized to a desired idling speed.
In order to obtain stability of the control, the feedback control is preferred to be carried out utilizing integral control. (See, U.S. Pat. No. 4,219,000, for example.)
The idling speed is sometimes increased for some reason while the idling speed control is stabilized. For example, when icing occurs around the throttle valve, the amount of the intake air is reduced to lower the idling speed and accordingly the control system successively generates signals for increasing the idling speed. If the icing is meanwhile released in such a state, the amount of the intake air is abruptly increased and the idling speed becomes abnormally high. It is not desirable from the viewpoint of safety and the feeling of the driver that the idling speed dwells in such a high speed zone for a long time.
Thus there has been proposed an idling speed control system having a fail-safe mechanism in which the feedback control of the idling speed is interrupted when the idling speed becomes higher than a predetermined value for some reason and the throttle valve is positively moved in the closing direction. See Japanese Patent Publication No. 53(1978)45861.
However, if the method in which the feedback control of the idling speed is interrupted in the fail-safe manner when the idling speed becomes higher than a predetermined value is simply applied to the idling control system in which the difference between the actual idling speed and a desired idling speed is integrated and an adjusting valve for controlling the amount of the intake air is controlled according to the integrated value to control the idling speed, the integrating function for integrating said difference is interrupted when the idling speed accidentally becomes high and the last integrated value which is the value obtained by integration immediately before the idling speed becomes higher than the predetermined value is held. Therefore hunting in which the rotational speed of the engine goes up and down around the predetermined value continues for a while when the rotational speed returns below the predetermined value and the feedback control is carried out again, and the rotational speed can not be rapidly equalized to the desired idling speed.